Regulatory T-cells in Immune Tolerance and Organ-specific Autoimmunity

Autoimmune diseases reflect a breakdown in self-tolerance and are a leading cause of premature death and disability. These chronic diseases require costly treatment with non-specific immunosuppressants with adverse side effects and have no cure. Understanding the pathogenic mechanisms of autoimmunity is key to designing effective therapeutics. CD4+Foxp3+ regulatory T (Treg) cells are under investigation as a potential therapy. Treg cells are critical for regulating various types of responses, such as allograft rejection, allergy, inflammation, infection, and autoimmunity. We employ the DEREG mouse model, expressing a diphtheria toxin receptor under control of a foxp3 locus to study the requirement of Treg cells in organ-specific autoimmunity. Treg cell-depleted C57BL/6 and/or C57BL/6 x A/J FI (B6AF1)-DEREG mice develop autoimmune gastritis (AIG), autoimmune orchitis (EAO), and autoimmune ovarian disease (AOD).
Our study indicates that transient Treg cell depletion results in long-lasting AIG associated with H+K+ATPase and intrinsic factor autoantibodies (Ab). Although functional Treg cells emerge over time during AIG, effector T cells rapidly become resistant to Treg cell-mediated suppression. Whereas previous studies implicated dysregulated Th1 cell responses in AIG pathogenesis, eosinophils have been detected in gastric biopsies from patients with AIG. Indeed, AIG in DEREG mice is associated with strong Th2 responses, including IgG1 autoAb, elevated serum IgE, increased Th2 cytokine production, and eosinophil expansion in the stomach-draining lymph nodes. Additionally, the stomachs exhibit severe mucosal and muscular hypertrophy, parietal cell loss, mucinous epithelial cell metaplasia, and massive eosinophilic inflammation. Notably, AIG is significantly ameliorated in IL-4- or eosinophil-deficient mice.
We also investigate Treg cell control of autoimmune responses to meiotic germ cell antigens (MGCA), which are responsible for post-vasectomy (Vx) autoimmunity and some cases of infertility. The prevailing view posits that MGCA are completely sequestered behind the blood-testis barrier. Thus, they are considered highly immunogenic and non-tolerogenic. We discovered that some murine MGCA are non-sequestered (NS-MGCA), and they egress from normal seminiferous tubules, react with circulating Ab, and form immune complexes. We identify one mechanism of NS-MGCA egress from the seminiferous tubules and show that these Ag maintain Treg cell-dependent physiological tolerance. In contrast, sequestered MGCA (S-MGCA) are non-tolerogenic. Significantly, DEREG mice with transient Treg cell depletion develop EAO and produce autoAb exclusively targeting NS-MGCA, which is distinct from the post-Vx autoAb targeting S-MGCA. Therefore, the sequestration status and tolerogenicity of MGCA influence target Ag selection in EAO and possibly influence tumor immunogenicity, as both sets of Ag are expressed as cancer/testis Ag in human tumors.
Furthermore, we describe two new models of AOD after Treg cell depletion early or late in life. The immune system of juveniles greatly differs from that of adults, which can impact their susceptibility to infection, response to vaccines, and development of immune-mediated disorders. Some patients with premature ovarian failure have serum ovarian autoAb, ovarian inflammation, and frequent occurrence of other autoimmune diseases, supporting an autoimmune basis for the disease. In humans, early-onset ovarian failure is more severe than late-onset disease. Thus, the age at onset of disease may influence AOD severity. Indeed, Treg cell depletion in adult DEREG mice results in Th2-dominant eosinophilic AOD with IgG1 autoAb targeting a unique B cell epitope in the oocyte zona pellucida 3 protein. In contrast, Treg cell-depleted juvenile mice develop severe and accelerated Th1-dominant and NK/NKT cell-dependent granulomatous AOD and ovarian atrophy. Their low incidence and titer autoAb responses target the oocyte cytoplasm, zona pellucida, and/or interstitial cells. The striking ontogenetic differences in AOD are not replicated in AIG in the same mice, as AIG in both juvenile and adult Treg cell-depleted mice are similarly Th2-dominant and autoAb targeting the same gastric Ag.
Collectively, the observed development of spontaneous autoimmunity in these otherwise lymphoreplete animals indicates that Foxp3+ Treg cells maintain physiological tolerance to clinically relevant gastric autoAg, NS-MGCA, and ovarian autoAg. In AIG and AOD, the T cell response was strongly Th2-biased after transient Treg cell depletion, supporting the significance of a Th2-dominant mechanism in autoimmune disease pathogenesis. Treg cells control tolerance at all ages, but the resulting pathogenic autoimmune responses after loss of Treg cell-mediated tolerance are age-dependent and organ-dependent. Understanding these distinct pathogenic mechanisms will aid in the identification of therapeutic targets and development of appropriate therapies.